I took a GY6 oil drain screen cap (S200 and SYM HD 200 use a standard GY6 cap/ring/spring, I have replaced both with no leaks) to match up the O ring, which tends to go bad, at least on my bikes, after a change.
While the ring is supposedly metric (38X30 or something) the best match was SAE:
ID 1 3/16
OD 1 7/16
W 1/8
I did find metric ones that "seemed" to fit the specs, but they were too loose in the rim in the cap. They come in 3mm and 4mm width, but 3 was too thin, 4 was too thick. Probably takes 3.5mm, but the ACE hardware did not carry that size.
The SAE above is listed as "industry standard #217", and I am certain you can get one at any good hardware store. The fit is perfect, just like the GY6 "original" requires a small blade to dig it back out of the rim once seated (or I just have spastic fingers). I am certain they will work. Cost was .55 each.
I always keep spares on hand, as every time I clean the screen it seems that I get a leak from the O Ring. Better to just change it with the oil.
